Presses are used in a huge variety of applications across many industries. Typically, the primary function of the press is to apply a force to achieve a result. Some examples include assembling components together by forcing one into another or deforming a material to match the shape of a die or mold. Sometimes, the pressing force must be accurately monitored over a large range of force, such as from zero to 25,000 pounds. This invention relates to force monitoring during the pressing process, and the device disclosed applies equally to presses applying force using servo electric, hydraulic, and pneumatic force generation.
Feedback from a force transducer (e.g. a strain gage based load cell) is typically analog in nature and must be converted to digital in order to be read by a digital computer. The voltage range is typically 0-10 volts for forces from 0 pounds to the maximum force rating of the load cell. The higher the force rating of the load cell, the lower the resolution (force granularity) becomes. As a result of this granularity, precision is lost when pressing in a lower force range.
In any load cell application, the force resolution and accuracy is directly proportional to the maximum force the load cell can measure. Thus, lower ultimate force will always result in better resolution and accuracy. Electrical noise on the A/D circuit (there will always be at least a few millivolts of noise) produces an apparent force proportional to the maximum load cell capacity. Additionally, resolution equals the maximum load cell force divided by the A/D resolution. Higher ultimate force results in more coarse force resolution.
The present invention overcomes this drawback by employing a second load cell with a lower range that works in tandem with the higher force load cell.